This invention relates to a liquid quantity sensor of the type electrically detecting the quantity of liquid existing in a container by utilizing a variation in the capacitance between two metal plates vertically disposed in the container. For example, the liquid container is a fuel tank in a vehicle.
As is known, it is possible to detect the quantity of a liquid existing in a container by fixedly and vertically disposing two adequately spaced flat and parallel, or cylindrical and concentrical, electrode plates in the container and measuring the capacitance between these two electrode plates. This method is based on the fact that most liquid materials have dielectric constants considerably different from the dielectric constant of air. As the liquid level in the container rises or lowers, the submerged fraction of the effective surface area of the capacitor formed by the two electrode plates increases or decreases with a corresponding change in the value of capacitance between the two electrode plates. By suitably designing the configuration of the electrode plates it is possible to establish a practically linear relationship between the level or quantity of liquid existing in the container and the capacitance between the two electrode plates.
When a liquid container provided with a liquid quantity sensor of the described type is kept stationary there is little problem in detecting the liquid quantity. However, the situation is different when the liquid container is installed on a vehicle such as an automobile as a fuel tank. In this case difficulties arise in accurately detecting the quantity of fuel in the tank during running of the vehicle because the fuel tank inclines to various degrees and the fuel itself makes inclining and undulating movements. The difficulties are augmented by rapid and considerable changes in the fuel level attributed to the fact that usually the fuel tank is shallow relative to its lateral widths and has an asymmetric shape in plan view. With a view to resolving such difficulities it is usual to use a liquid quantity sensor having a plurality of sets of capacitor electrode plates which are disposed at suitably selected locations in the fuel tank and all connected in parallel. Since most fuel tanks for vehicles are provided with baffleplates which are fixedly disposed in each tank so as to divide the interior of the tank into several sections, which are communicating with one another, for the purpose of suppressing noise-producing undulations of the fuel during running of the vehicle, it is also usual to utilize the baffleplates for mechanically supporting the capacitor electrode plates of the liquid quantity sensor.
Irrespective of the configuration and arrangement of the capacitor electrode plates, it is very important for accurate detection of the liquid quantity to keep an accurate and uniform distance between each pair of electrode plates. My copending U.S. Patent Application Ser. No. 330,782 filed Dec. 14, 1981, which is incorporated herein by reference, discloses an improved liquid quantity sensor of the capacitor type in which the accuracy and uniformity of the distance between each pair of electrode plates (one of which is a baffleplate) are assured by a plurality of insulating spacers which have an accurately determined length or thickness and are inserted between the two electrode plates.
However, there is yet another matter that adversely affects the accuracy of liquid quantity sensors of the capacitor type. In many cases current liquid quantity sensors of this type applied to vehicle fuel tanks have at least one pair of capacitor electrode plates that are angled or bent to provide a vertical bend with a view to reducing errors in the measurement attributed to irregular movements of fuel in the tank. In industrial production of such liquid quantity sensors it is difficult to keep a uniformly constant distance between the paired two electrode plates in the bent region thereof, and therefore the mass-produced liquid quantity sensors of the same design often indicate somewhat different capacitance values for a given quantity of liquid in a container such as a fuel tank.